feat(config): add validation.on-create and validation.on-sync config options (bd-t7jq)

Add .beads/config.yaml support for template validation settings:
- validation.on-create: warn|error|none (default: none)
- validation.on-sync: warn|error|none (default: none)

When set to "warn", issues missing required sections (based on type) show
warnings but operations proceed. When set to "error", operations fail.

Implementation:
- Add validation keys to YamlOnlyKeys in yaml_config.go
- Add defaults in config.go
- Wire up bd create to check validation.on-create config
- Wire up bd sync to run validation before export
- Add tests for config loading
- Update CONFIG.md documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/create.go

@@ -166,12 +166,26 @@ var createCmd = &cobra.Command{
 			estimatedMinutes = &est
 		}
 
-		// Validate template if --validate flag is set
+		// Validate template based on --validate flag or config
 		validateTemplate, _ := cmd.Flags().GetBool("validate")
 		if validateTemplate {
+			// Explicit --validate flag: fail on error
 			if err := validation.ValidateTemplate(types.IssueType(issueType), description); err != nil {
 				FatalError("%v", err)
 			}
+		} else {
+			// Check validation.on-create config (bd-t7jq)
+			validationMode := config.GetString("validation.on-create")
+			if validationMode == "error" || validationMode == "warn" {
+				if err := validation.ValidateTemplate(types.IssueType(issueType), description); err != nil {
+					if validationMode == "error" {
+						FatalError("%v", err)
+					} else {
+						// warn mode: print warning but proceed
+						fmt.Fprintf(os.Stderr, "%s %v\n", ui.RenderWarn("⚠"), err)
+					}
+				}
+			}
 		}
 
 		// Use global jsonOutput set by PersistentPreRun